The present invention relates to a focussing solenoid. It also relates to a process for manufacturing same.
The present invention relates to the field of focussing an electron beam in an electron tube. In an electron tube, it is known to provide a focussing device whose purpose is to oppose the natural divergence of the electrons of the beam. This device often creates a magnetic field directed along the axis of the tube and causing the rotation about this axis of the divergent electrons. In some applications, the requirements for concentrating the beam are particularly severe. This is the case for example with electron tubes intended for taking pictures, electron microscopes and ultrahigh frequency electron tubes.
In picture taking tubes, for focussing the beam it is known to use a solenoid, disposed around the tube and formed by an insulated metal wire, made from enamelled copper for example, which is wound in a helical form around an insulating sleeve, made for example from epoxy glass. The turns of the helix are jointing and the solenoid comprises several superimposed layers of wire, wound in the same direction around the sleeve.
Thus, for example, on a sleeve of 3 cm diameter and 8 cm long, are superimposed 20 layers of 300 turns each using a wire having a diameter of 25/100th mm.
The wire is wound on the sleeve using a winding machine comprising a wire guide and the laying of the turns is controlled by hand. After winding one layer, the next layer is wound thereabove without cutting the wire but reversing the slope of the turns with respect to the axis. Thus, the winding direction of the wire about the sleeve is always the same and the current always flows in the same direction which allows, in a first approximation, a uniform magnetic field to be obtained parallel to the axis. Two flanges fixed at the ends of the sleeve hold the wire layers in place.
The problem which arises is that the distribution of the turns of the different layers is not even and then the type of focussing solenoid which has just been described gives poor characteristics to the picture taking tube with which it is associated.
Thus, this solenoid causes a parasite deflection of the image which results in "frame offset". It also causes an error of orthogonality. The rapid return of the beam controlled by the deflection coils forms a parasite which is transmitted to the target by the focussing solenoid which disturbs the video signal. Finally, it can be observed that the modulation rate varies from one solenoid to another and also a lack of homogeneity of resolution of the tube can be observed.